If you are running Go 1.5.x you can disregard this message.
We have just issued Go 1.4.3 to address four security-related issues.
The issues were reported in Go's net/http package. They affect programs using that package to proxy HTTP requests. We recommend that all users upgrade to Go 1.5, which fixes these issues. For users unable to upgrade to Go 1.5, we have released version 1.4.3, which is based on Go 1.4.2 plus fixes for these issues. Affected Go programs—those that use the net/http package as a proxy server—must be recompiled with Go 1.5 or Go 1.4.3 to receive the fixes.
The CVE issue descriptions and fixes are linked below.
CVE-2015-5739
"Content Length" treated as valid header:
CVE-2015-5740
Double content-length headers does not return 400 error:
CVE-2015-5741
Additional hardening, not sending Content-Length w/Transfer-Encoding,
Closing connections: